06/16 Phase 6 foundation: admin blueprint, audit log, user management

Adds the admin backend foundation: AuditLog + Setting models/migration,
an admin-only dashboard with KPI cards, and user management (search,
ban/suspend/activate, tier override, trust adjust) with audit logging
on every write action. Smoke suite extended to 64 checks.

app/models/audit.py

@@ -0,0 +1,24 @@
+"""Append-only audit trail for admin write actions."""
+from datetime import datetime
+from sqlalchemy import JSON
+from app.extensions import db
+
+
+class AuditLog(db.Model):
+    __tablename__ = "audit_logs"
+
+    id = db.Column(db.BigInteger().with_variant(db.Integer, "sqlite"),
+                   primary_key=True, autoincrement=True)
+    actor_id = db.Column(db.BigInteger().with_variant(db.Integer, "sqlite"),
+                         db.ForeignKey("users.id"), nullable=True, index=True)
+    action = db.Column(db.String(60), nullable=False, index=True)
+    target_type = db.Column(db.String(40), nullable=False)
+    target_id = db.Column(db.BigInteger().with_variant(db.Integer, "sqlite"),
+                          nullable=True, index=True)
+    meta = db.Column(JSON, nullable=True)
+    created_at = db.Column(db.DateTime, default=datetime.utcnow, nullable=False)
+
+    actor = db.relationship("User", backref=db.backref("audit_logs", lazy="dynamic"))
+
+    def __repr__(self):
+        return f"<AuditLog {self.action} {self.target_type}:{self.target_id}>"